The invention relates to a method of recording a surface relief microstructure in a record medium.
Surface relief microstructures are used to create diffraction and holographic effects which find particular application in security features for use on documents of value such as identity cards, banknotes, and the like.
Conventionally, such a microstructure is created in two stages. In a first stage, a H1 device is created by causing interference between a reference laser beam and an object beam which has impinged on an object. The H1 is then used subsequently upon exposure by a suitable conjugate beam to generate a real image which is recorded on a H2 record medium.
This process is relatively cumbersome to achieve and recently digital diffraction, dot-matrix or digital holographic printers have become available for producing holographic master resists for the security industry. Dot-matrix holography is described in U.S. Pat. No. 6,043,913. In this process, individual dots are exposed by causing two laser beams to interfere with each other to generate interference fringes which are recorded. Typically, this is achieved by causing the two beams to impinge on respective portions of a focussing lens which then focuses the beams towards each other into a focussing region where the record medium is located.
The problem with this known technique is that in order to achieve spectral purity it is desirable to decrease the cross-sectional area of each beam but the consequence of this is that the size of the resultant spot expands.
In accordance with the present invention, a method of recording a surface relief microstructure in a record medium comprises:
i) causing a coherent reconstruction or replay beam to impinge on a holographic optical element assembly, the holographic optical element assembly including a holographic optical element (HOE), the assembly causing selected portions of the beam to interfere at a focal region after diffraction by the holographic optical element so as to reconstruct a real image of an aperture previously used to construct the HOE;
ii) locating a record medium at the focal region so that the real image is recorded; and,
iii) causing relative movement between the record medium and the interfering beam portions and repeating steps i) and ii) so that the real image is recorded at a plurality of locations or pixels on the record medium.
With this invention, the cross-section of the beams which interfere to produce the diffraction pattern at the focal region is set independently from the resultant spot size by utilizing a HOE as the focussing element. It is a property of a HOE that the spot size is invariant with exposing beam size. This then enables the shape of the resultant spot to be selected as desired by suitably defining the shape of the aperture used to create the HOE.
In order to select the beam portions, the holographic optical assembly preferably further includes an aperture mask. This mask could be in the form of a diffractive optical element (DOE) but is preferably in the form of a spatial light modulator such as a liquid crystal display.
The beam portions may impinge on the record medium in a symmetrical manner but in a particularly preferred approach the holographic optical element is constructed and illuminated such that the beam portions impinge on the record medium at angles on the same side of a normal to the record medium. Usually, the beam portions are in substantially the same plane which also contains a normal passing through the point of impingement. Pixels which are imaged with just two beam portions will have the general quality that they resemble conventional dot matrix pixels. If, however, these beams are both on the same side of the normal in the same plane, or both tilted from the same side of that plane, then they will have the capability in either case to produce a xe2x80x9cblazed gratingxe2x80x9d. Pixels addressed by more than two beams would tend to involve individual beam portions which do not lie in the same plane, and would thus contain recorded fringe structures of great complexity akin to those of a conventional hologram in that particular pixel. A xe2x80x9cblazedxe2x80x9d grating is capable of very high diffraction efficiency levels up to 100% in the first order.
The invention also leads to the possibility of providing a set of interchangeable HOE""s allowing the user to choose between a range of edge profiles and shapes for the image pixels of the final, recorded structure, typically a hologram. In some cases, only two beam portions will be used to reconstruct the real image but in other cases more than two beam portions could be used. Microstructures produced by simple two-beam interference can be referred to as diffraction gratings while pixels containing fringed structures produced by different multiple (i.e. more than two) beam portions from a range of directions are true xe2x80x9chologramsxe2x80x9d.
One of the novel features of the method is that the actual dot forming each pixel is comprised of the interference between holographic real images in or near their common focal plane, rather than being formed at the point of intersection of two independently apertured laser beams described in other equipment which is commercially available. The introduction of a level of diffusion in the object beam would lead to the appearance of laser speckle within the resulting second generation recording, and although deleterious to the final image clarity and efficiency, would further illustrate the true holographic nature of the recording.
As mentioned above, the resultant microstructure can be used in a variety of applications but particularly for securing documents and other articles against fraudulent reproduction and counterfeiting. These include documents of value including banknotes, cheques, bonds, traveller cheques, stamps, certificates of authenticity, high value packaging goods and vouchers and the like.